The present invention relates to a molded part having an insert therein, specifically to a structure of a molded part integrated with plural metallic terminals by way of insert molding and its production method.
It is required to make the thickness of resin as uniform as possible for making the molded parts in order to prevent the degradation in fluidity and surface property of the resin. However, if the molded part has a portion with an increased thickness, as this increased thickness part has a larger amount of contraction due to the molding process in comparison with the other portions in the molded part, unfavorable void generation inside such an increased thickness portion may be caused when a thickness is larger than a designated limitation value. In case of molded parts having an insert therein, there is such a problem that the contraction of the increased thickness part and the voids generated inside it may increase the gap developed in the interface between the insert and the molded part.
In general, there is a method for molding the resin used for the increased thickness part in a two-layer structure in order to solve the above described problem. In this method, sub parts having a required size are formed in a preliminary molding process, and then, those pre-molded parts are used as inserts, and finally, those inserts are integrated with over-molded parts by way of multiple molding.
In the molded part formed by using the inserts, in the progress of mass production of a wide variety of goods, downsizing and sophisticated configuration of an insert due to higher functionality of parts, there is a limit for increasing the productivity only with a single molding process. In order to solve this problem, sub parts are formed in a preliminary molding process, and then, those pre-molded parts are inserted as insert members, and finally, those parts are integrated with over molded parts by way of multiple molding so that the productivity and the yield rate of the product may be increased. In addition, in terms of higher productivity and low-cost in the molding process, thermoplastic resin is often used as the molded material and an injection molding method is used widely for the molding method in general.
In case of a multiple molding process, a stress due to resin contraction at the over-molding process is applied to the pre-molded part used as an insert, which results in an internal distortion inside the pre-molded part. In addition, a heating process for integrating and molding at the over-molding process may give rise to resin contraction of the pre-molded part. This resin contraction mechanism is different from the contraction of the single body of the pre-molded part and depends on the type and shape of the over-molded part, which may result in such a problem as increasing the gap developed in the interface between the insert and the pre-molded part.
In order to solve the above described problem, in one method, an adhesive material such as epoxy resin is coated and hardened on the exposed surface of the insert after the over-molding process in order to seal the interface between the insert and the pre-molded part, and to further fix firmly the insert and the pre-molded part.
In Japanese Patent Laid-Open No. 7-142817 (1995), a method is disclosed such that epoxy resin composed of an adhesive material having a heat hardening property is coated on an insert for the cover film, and the heat treatment is applied after insert molding in order to bond the insert and the insert molding resin.
This pre-mold process is applied to the insert molding of plural metallic terminals as the insert used for electrical connection.
However, for the same reason described above, in a multiple molding process, a stress due to resin contraction in the over-molding process is applied to the pre-molded part inserted as an insert, which results in an internal distortion inside the pre-molded part. In addition, the heating process for integrating and molding during the over-molding process may give rise to resin contraction of the pre-molded part.
This resin contraction mechanism is different from the contraction of the single body of the pre-molded part and depends on the type and shape of the over-molded part. In applying an insert molding of plural metallic terminals as the insert used for electrical connection, this resin contraction may result in such a problem as increasing the gap developed in the interface between the metallic terminal and the pre-molded part.
In a wire-bonding process in assembly of the product, as such internal distortion and the gap so developed in the molding process disperse and absorb the vibration associated with the bonding process, bonding failure may result.
In order to solve the above described problem, another method provides that an adhesive material such as epoxy resin is coated and hardened on the region of bonding part of the metallic terminal after the over-molding process in order to fix firmly the metallic terminal and the pre-molded part.
As for another solution, the metallic terminal is inserted and pre-molded by using heat-hardening resin, which provides a small amount of post-contraction and is less subject to heat problems in the over-molding process, for the pre-molded part.